In recent years, along with development of the digital technology, electronic devices such as portable information devices and information home appliances have increasingly higher-level functionalities. There is thus a higher demand on nonvolatile memory elements for an increase in capacity, a reduction in power for writing, an increase in speed for writing/reading, and a longer operating life.
In response to such a demand, it is said that there is a limit on the miniaturization of existing flash memories using floating gates. On the other hand, a nonvolatile memory element using a variable resistance layer as a material of a memory unit (i.e., a variable resistance memory) can be composed of a simple-structured memory element represented by a two-terminal variable resistance element, which therefore lays high expectations for further miniaturization, increase in speed, and reduction in power consumption.
The variable resistance layer which is used as a material of the memory unit will have resistance changing in value from high resistance to low resistance or from low resistance to high resistance by input of electric pulses or the like, for example. In this case, it is necessary that two values of low resistance and high resistance be clearly distinguished, a change between low resistance and high resistance be stable at high speed, and these two values be held in a nonvolatile manner. As an example of this variable resistance element, a nonvolatile memory element using stacked transition metal oxides with different oxygen content atomic percentages for the variable resistance layer has been proposed. There is a disclosure that, in this variable resistance element, a change in resistance is stabilized by selectively causing an oxidation-reduction reaction at an electrode interface in contact with the variable resistance layer with a high oxygen content atomic percentage (see Patent Literature 1, for example).
FIG. 23 shows a conventional variable resistance nonvolatile memory element 50 including a variable resistance element 55. A first line 101 is formed on a substrate 100, and a first interlayer insulating layer 102 is formed so as to cover this first line 101. A first contact plug 104 is formed which penetrates the first interlayer insulating layer 102 and is connected to the first line 101. The variable resistance element 55 made up of a lower electrode 105, a variable resistance layer 106, and an upper electrode 107 is formed on the first interlayer insulating layer 102 so as to cover the first contact plug 104. A second interlayer insulating layer 108 is formed so as to cover the variable resistance layer 55, and a second contact plug 110 penetrating the second interlayer insulating layer 108 connects the upper electrode 107 and a second line 111. The variable resistance layer 106 has a laminated structure of a first variable resistance layer 106x and a second variable resistance layer 106y and comprises transition metal oxides of one kind. The transition metal oxide of the first variable resistance layer 106x has a higher oxygen content atomic percentage than the oxygen content atomic percentage of the transition metal oxide of the second variable resistance layer 106y. 
With such a structure, applying voltage to the variable resistance element 55 will result in the most of the voltage being applied to the first variable resistance layer 106x that is high in oxygen content atomic percentage and has a larger resistance value. Moreover, near this interface, there is abundant oxygen which can contribute to the reaction. Thus, the oxidation-reduction reaction selectively occurs at the interface between the upper electrode 107 and the first variable resistance layer 106x so that the resistance can stably change.
Non Patent Literature 1 discloses a nonvolatile memory including 1T1R (meaning one transistor and one variable resistance element which are connected in series) memory cells each of which uses a transition metal oxide for the variable resistance element. According to the disclosure, a thin film of the transition metal oxide usually serves as an insulator and has a conductive path formed therein by initialization for causing a pulsed change in the resistance value. The conductive path allows switching between a high resistance state and a low resistance state. Here, “initialization” refers to a process of changing a manufactured variable resistance element or variable resistance nonvolatile memory element into a state in which the high resistance state and the low resistance state can be reversibly changed according to applied voltage (or polarity of applied voltage). Specifically, “initialization” refers to a process of applying higher voltage than write voltage to the manufactured variable resistance element or variable resistance nonvolatile memory element which has a very large resistance value. This initialization not only places the variable resistance element or variable resistance nonvolatile memory element in a state in which the high resistance state and the low resistance state can be reversibly changed, but also lowers the resistance value of the variable resistance element or variable resistance nonvolatile memory element.
FIG. 24 shows characteristics of dependency of a thickness of the transition metal oxide relative to voltage for initialization, shown in Non Patent Literature 1. Characteristics of the transition metal oxide (TMO) of four kinds: NiO, TiO2, HfO2, and ZrO2 are shown, and required voltage for initialization depends on the kind of the transition metal oxide and is higher as the thickness of the transition metal oxide increases. Thus, in order to lower the voltage for initialization, it is preferable to reduce the thickness of the transition metal oxide.